1. Field of the Invention
The present invention relates to a control apparatus for a resistance welding machine, especially to a control apparatus for a spot welding machine.
2. Prior Art
In a resistance welding machine used for lap welding of steel plates or the like, the three principal parameters to stabilize the welding quality are a welding current, an energizing time, and an electrode pressurizing force. It is generally known that these parameters are feedback controlled instead of by the dial setting of recommendable conditions for each base metal based on expertise and experiences.
Enhancement of the welding quality has been recently requested. To satisfy such a requirement, Unexamined Japanese Patent Application No. 4-178275, published in 1992, discloses a technology for controlling a welding operation by directly monitoring a nugget, since the nugget growth reflects the result of the welding operation.
Hereinafter, the control apparatus of a conventional resistance welding machine disclosed in the Unexamined Japanese Patent Application No. 4-178275 will be explained with reference to the accompanied drawing.
In FIG. 4, reference numeral 21 represents a power source unit for a resistance welding machine, and reference numeral 22 represents a controller for the welding power source unit 21. Reference numeral 23 represents a welding current detecting device. Reference numeral 24 represents a secondary conductor. Reference numeral 25 represents a lower arm. Reference numeral 26 represents a base metal to be welded. Reference numeral 27 represents a pair of electrodes. Reference numeral 28 represents a pressurizing cylinder.
Reference numeral 29 represents an upper arm. Reference numeral 30 represents an electropneumatic proportional valve. Reference numeral 31 represents a pressure sensor. Reference numeral 32 represents a controller for the electropneumatic proportional valve 30. Reference numeral 33 represents a cable which detects an interelectrode voltage (i.e. a voltage between electrodes 27). Reference numeral 34 represents a distance detector which detects an amount of shift of electrodes 27.
Reference numeral 35 represents a signal processing section which acts as a hardware circuit for processing signals representing the electrode pressurizing force, the electrode shift amount, the interelectrode voltage and the welding current. Reference numeral 36 represents an estimating section which estimates a nugget size and a heat input density. And, reference numeral 37 represents a control signal generating section which generates control signals for the welding current and the electrode pressurizing force.
The above-described control apparatus of the conventional resistance welding machine will be explained with reference to a flow chart shown in FIG. 5. First, the plate thickness of base metal 26, the number of metal plates to be lapped or stacked, and the material information of base metal 26 are respectively entered (step 101) in the control apparatus. Next, a welding operation is started (step 102). Prior to an energizing operation, electrodes 27 are pressurized to confirm the total thickness of metal plates. Then, a relationship between the actual pressurizing force and the shift amount of electrodes 27 is measured. Based on the measuring result, an electrode pressurizing force value is set to an appropriate value to make the metal plates fit aginst each other sufficiently (step 103).
Referring to the lapping number of metal plates, a standard heat input density pattern and a standard energizing diameter increase pattern are selected respectively (step 104). The standard heat input density pattern and the standard energizing diameter increase pattern are both determined in advance in accordance with numerical calculations and experiments. Selection of these two standard patterns is essential for realizing an effective control of the variation of the heat input density and the increase of the energizing diameter during the actual welding operation. More specifically, both the heat input density and the energizing diameter are controlled to be equalized to the values designated by the selected standard patterns.
Next, the energizing operation is started (step 105). Then, a momentary change of a nugget diameter is monitored through a simulation of numerical calculations while performing a heat input density control later described (step 112). When the estimated nugget diameter becomes larger than a required nugget diameter (step 113), the energizing operation is terminated (step 114). Through this operation, it becomes possible to realize an excellent welding portion with reliability.
Next, the simulation of numerical calculations for obtaining the nugget diameter and the heat input density control will be explained. An interelectrode voltage and a welding current value are detected during the welding operation after starting the energizing operation (step 106). The detected values of the interelectrode voltage and the welding current are substituted into the following equation (1) to calculate the energizing diameter (step 107). ##EQU1## where"dc" is a representative energizing diameter of a welding portion,".rho.m" is a mean specific resistance of the welding portion,"A" is a correction coefficient relating to current spread,".SIGMA.hi" is a total plate thickness, "Rtip" is an interelectrode resistance (=Vtip/I, when "Vtip" is an interelectrode voltage and"I" is a welding current), and "R0" is an electrode resistance.
In the above-described equation (1), the mean specific resistance".rho.m" of the welding portion is determined based on a mean temperature in the welding portion. When the energizing operation is started (t=0), the mean specific resistance ".rho.m" is equal to a specific resistance ".rho.m0" at the room temperature. Temperature change during a tiny time interval .DELTA.t can be regarded as negligible. Next, the energizing diameter thus calculated and the detected welding current value are substituted into the following equation (2) to estimate a temperature distribution to be obtained after a time elapse of .DELTA.t. ##EQU2## where "c" is a specific heat, ".sigma." is a density, "K" is a heat conductivity, ".delta." is a current density (.tbd.AI/(.pi..multidot.dc.sup.2 /4)), "T" is a temperature, "t" is a time, "x" is a distance in the direction of plate thickness, and ".differential." is a partial derivative symbol.
From this temperature distribution, ".rho.m1" is obtained as a mean specific resistance to be obtained after a time elapse of .DELTA.t. By substituting ".rho.m1" into the equation (1), an energizing diameter to be obtained after a time elapse of .DELTA.t can be calculated. In this manner, momentary values of the energizing diameter, temperature distribution, and heat input density are successively obtained during a duration from start of the energizing operation to a certain time (step 110). Furthermore, a correct nugget diameter can be estimated by taking into account a heating start-up delay time at each radial position. Then, the welding current and the electrode pressurizing force are controlled to equalize the energizing diameter and heat input density obtained here to the preferable standard heat input pattern and preferable standard energizing diameter increase pattern selected in step 104 (step 111).
A detection of an electrode shift amount (step 108) is necessary to confirm and correct (step 109) an error of the energizing diameter calculated in step 107, which is likely to be caused in the initial stage of the energizing operation. The detected electrode shift amount is substituted into the following equation (3) to calculate a mean temperature of the welding portion. Then, the mean temperature thus obtained is used to correct the mean temperature distribution at the present time which is previously obtained. ##EQU3## where "Tm" is a mean temperature of the welding portion, ".DELTA.1" is an electrode shift amount, ".alpha.m" is a mean value of linear expansion coefficient, and "B" is a proportional constant.
However, according to the above-described conventional arrangement, it is necessary to successively identify the mean temperature and the energizing diameter of the welding portion during the welding operation, and it is complicated to determine the standard energizing diameter increase pattern and the standard heat input density pattern to compare them with the identified energizing diameter and the heat input density obtained from this energizing diameter. Furthermore, it is necessary to perform an adaptive control in real time for adjusting the welding current and the electrode pressurizing force based on the comparison result of the energizing diameter and the heat input density in connection with the standard energizing diameter increase pattern and the standard heat input density pattern. Therefore, the control apparatus becomes complicated and expensive.
Furthermore, in the resistance welding machine used for lap welding of steel plates or the like, increasing a heat input amount to a welding portion is important to increase the welding strength. The factors determining the heat input amount are a welding current, an energized portion resistance, and an energizing time. Especially, when the number of welding points is increased, the tip of an electrode is worn and deformed. This leads to deterioration of the welding strength. Thus, it becomes necessary to further increase the heat input amount to the welding portion. However, increasing the heat input amount provokes generation of expulsion & surface-flash. It is well known that, when the expulsion & surface-flash is once generated, the welding strength is extremely reduced and the appearance or configuration of the welding portion is terribly worsened.
From the enhancement of the welding quality, the recent development of the welding technology makes it possible to obtain a sufficient size of a nugget while suppressing the generation of the expulsion & surface-flash to a minimum level. For example, it is effective to intentionally reduce the welding current during a start-up period of a welding operation and also during the second-half period of the welding operation, because the expulsion & surface-flash tends to be caused in these specific durations. Meanwhile, an increase is allowed in the welding current largely during an intermediate period of the welding operation, because the expulsion & surface-flash seldom occurs in this intermediate duration.
In view of the foregoing, a pattern welding current control and a constant-power welding method are already proposed, as disclosed in Unexamined Japanese Patent Application No. 63-180384, published in 1988. Furthermore, there is a method of stopping the energizing operation as soon as the generation of the expulsion & surface-flash is detected by the sudden change of a pressurizing force or a welding current as disclosed in Unexamined Japanese Patent Application No. 1-241385, published in 1989.
Moreover, for a plurality of welding points which are continuously welded, there is a method of varying a welding current for the next welding point when an expulsion & surface-flash is detected during a welding operation for a certain welding point, as disclosed in Unexamined Japanese Patent Application No. 5-337655, published in 1993.
However, according to the above-described conventional technologies, the following problems arise. According to the pattern welding current control or the constant-power welding method, the generation rate of the expulsion & surface-flash can be reduced to a certain degree compared with the constant-current welding method. However, it is inherently impossible to obtain a nugget having a maximum strength without causing any expulsion & surface-flash. Meanwhile, according to the method of stopping the energizing operation immediately upon detecting the generation of expulsion & surface-flash, it necessarily generates some expulsion & surface-flash. Furthermore, according to the method of varying the welding current for the next welding point when any generation of the expulsion & surface-flash is detected in a present welding operation, it is impossible to prevent the generation of the expulsion & surface-flash during the present welding operation. In short, there was no method capable of perfectly preventing the generation of the expulsion & surface-flash.